netdev - [v3 PATCH 2/9] rhashtable: Eliminate unnecessary branch in rht_key

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Message-Id: <E1Ya2kG-0004HI-9N@gondolin.me.apana.org.au>
Date:	Tue, 24 Mar 2015 00:50:20 +1100
From:	Herbert Xu <herbert@...dor.apana.org.au>
To:	"David S. Miller" <davem@...emloft.net>,
	Thomas Graf <tgraf@...g.ch>,
	Eric Dumazet <eric.dumazet@...il.com>,
	Patrick McHardy <kaber@...sh.net>,
	Josh Triplett <josh@...htriplett.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	netdev@...r.kernel.org
Subject: [v3 PATCH 2/9] rhashtable: Eliminate unnecessary branch in rht_key_hashfn

When rht_key_hashfn is called from rhashtable itself and params
is equal to ht->p, there is no point in checking params.key_len
and falling back to ht->p.key_len.

For some reason gcc couldn't figure out that params is the same
as ht->p.  So let's help it by only checking params.key_len when
it's a constant.

Signed-off-by: Herbert Xu <herbert@...dor.apana.org.au>
---

 include/linux/rhashtable.h |    8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/include/linux/rhashtable.h b/include/linux/rhashtable.h
index 18a6488..e316f10 100644
--- a/include/linux/rhashtable.h
+++ b/include/linux/rhashtable.h
@@ -199,8 +199,12 @@ static inline unsigned int rht_key_hashfn(
 	struct rhashtable *ht, const struct bucket_table *tbl,
 	const void *key, const struct rhashtable_params params)
 {
-	return rht_bucket_index(tbl, params.hashfn(key, params.key_len ?:
-							ht->p.key_len,
+	/* params must be equal to ht->p if it isn't constant. */
+	unsigned key_len = __builtin_constant_p(params.key_len) ?
+			   (params.key_len ?: ht->p.key_len) :
+			   params.key_len;
+
+	return rht_bucket_index(tbl, params.hashfn(key, key_len,
 						   tbl->hash_rnd));
 }
 
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